Magnetic recording medium

ABSTRACT

In a magnetic recording medium according to the present invention, a magnetic layer consisting of a metal magnetic material is formed on the upper surface of a non-magnetic support member, a protective layer is formed on the magnetic layer, and a back-coat layer is formed on the rear surface of the non-magnetic support member. In the present invention, in particular, the protective layer is formed by a thin film containing carbon as a main component and 5 to 20 atom % of hydrogen, and the back-coat layer is formed by a thin film containing carbon as a main component and 5 to 20 atom % of hydrogen, thereby making it possible to obtain a magnetic recording medium having excellent sliding characteristics and excellent durability.

FIELD OF THE INVENTION

The present invention relates to a magnetic recording medium in which aferromagnetic metal thin film and a protective layer are formed on onemajor surface of a non-magnetic support member, and a back-coat layer isformed on the other major surface.

BACKGROUND ART

In a magnetic recording medium used in a conventional video taperecorder (to be referred to as a "VTR" hereinafter) or the like, amagnetic layer is formed on one major surface (to be referred to as a"front surface" hereinafter) of a film-like non-magnetic support member,a protective layer is formed on the magnetic layer to protect it, and aback-coat layer for improving the characteristics of the magneticrecording medium is formed on the other major surface (to be referred toas a "rear surface" hereinafter) of the film-like non-magnetic supportmember.

As such a magnetic recording medium, a so-called coating type magneticrecording medium in which a magnetic coating material including amagnetic powdery material dispersed in a binder is coated on a film-likenon-magnetic support member and then dried to form a magnetic layer ispopularly used. In the coating type magnetic recording medium, theprotective layer or the back-coat layer is formed in the same manner asthat of the magnetic layer. More specifically, the protective layer orthe back-coat layer is formed in such a manner that a protective layermaterial or a back-coat layer material including carbon dispersed in abinder is coated on a non-magnetic member and then dried. At this time,the back-coat layer is generally coated with or includes a lubricantcontaining a fatty acid as a main component.

In contrast to this, with a demand of high-density magnetic recording, aso-called deposition type magnetic recording medium in which a magneticlayer is formed by depositing a ferromagnetic metal material on anon-magnetic support member has attracted attention.

Such a deposition type magnetic recording medium has the followingvarious advantages. That is, loss of a thickness in arecording/reproducing operation can be considerably decreased becausethe thickness of a magnetic layer in which saturation magnetization canbe increased with respect to the anti-magnetic force of the magneticlayer can be remarkably decreased, and a filling density of a magneticmaterial can be increased because binder serving a non-magnetic materialis not mixed in the magnetic layer.

As such a deposition type magnetic recording medium, the magnetic tapein which a Co₈₀ Ni₂₀ alloy is deposited on a film to form a magneticlayer was merchandized in 1989.

In the deposition type magnetic recording medium, as in a coating typemagnetic recording medium, a back-coat layer is formed in such a mannerthat a back-coat layer material obtained by dispersing carbon or thelike in a binder is coated on a non-magnetic support member and thendried.

However, in the deposition type magnetic recording medium, since themagnetic layer consists of a ferromagnetic metal material, when theback-coat layer is formed in a conventional manner, the followingproblem is posed.

That is, in the coating type magnetic recording medium, a lubricantcontaining a fatty acid as a main component is coated on or included inthe back-coat layer to decrease a friction coefficient of the back-coatlayer as described above. However, in the deposition type magneticrecording medium having the magnetic layer consisting of a ferromagneticmetal material, when a lubricant containing a fatty acid as a maincomponent is coated on or included in the back-coat layer, the lubricantcontaining the fatty acid as a main component grows the rust of theferromagnetic recording medium.

Therefore, in the deposition type magnetic recording medium, theback-coat layer cannot be coated with a lubricant containing a fattyacid as a main component, and cannot include the lubricant. For thisreason, the deposition type magnetic recording medium has the followingproblem to be solved. That is, the friction coefficient of the back-coatlayer is decreased without coating or including the lubricant containinga fatty acid as a main component on/in the back-coat layer.

A method of increasing the surface roughness Ra of the back-coat layerto degrading the surface properties may be considered to decrease thefriction coefficient of the back-coat layer. When the surface propertiesof the back-coat layer are degraded, and a tape-like magnetic recordingmedium is wound, the surface properties of the magnetic layer are alsodegraded depending on the unevenness of the back-coat layer. In thismanner, when the surface properties of the back-coat layer is degraded,the surface properties of the magnetic layer are also degraded due to aninfluence of the back-coat layer, and degradation of characteristics ina recording/reproducing operation or an increase in dropout is caused.Therefore, the method of degrading the surface properties of theback-coat layer to decrease the friction coefficient of the back-coatlayer has the above problem.

In the deposition type magnetic recording medium, the frictioncoefficient of the back-coat layer must be decreased without coating orincluding a lubricant containing a fatty acid as a main component on/inthe back-coat layer and without degrading the surface properties of theback-coat layer.

In the deposition type magnetic recording medium, the type of a binderused in formation of the back-coat layer is limited. More specifically,for example, a vinyl-chloride-based binder cannot be used becausechlorine causes a ferromagnetic metal material to rust. For this reason,in the deposition type magnetic recording medium, since the type of thebinder used in formation of a back-coat layer is limited, theappropriate composition of the binder cannot be easily founded.

In the deposition type magnetic recording medium, adhering forces amongthe magnetic layer, the protective layer, and the back-coat layerconsiderably change depending on the combination among the material ofthe magnetic layer, the material of the protective layer, and thematerial of the back-coat layer. For this reason, these materials mustbe appropriately determined. If these materials are not appropriatelyselected, for example, a back-coat layer may be partially peeled, or theback-coat layer may be separated from the support member in a widthdirection or a traveling direction to loss conductivity. Therefore, theback-coat layer may loss the effects of the back-coat layer.

The present invention has been made in consideration of the aboveconventional circumstances, and has as its object to mainly improve aprotective layer and a back-coat layer of a deposition type magneticrecording medium to prevent the deposition type magnetic recordingmedium from rust and improving the traveling and still characteristicsof the deposition type magnetic recording medium.

SUMMARY OF THE INVENTION

In order to achieve the above object, according to the presentinvention, there is provided a magnetic recording medium in which aferromagnetic metal thin film is formed on one major surface of anon-magnetic support member, a protective layer is formed on theferromagnetic metal thin film, and a back-coat layer is formed on theother major surface of the non-magnetic support member, characterized inthat the protective layer contains carbon as a main component and 5 to20 atom % of hydrogen, and the back-coat layer contains carbon as a maincomponent and 5 to 20 atom % of hydrogen.

In the magnetic recording medium, the protective layer contains carbonas a main component and 5 to 20 atom % of hydrogen, and the back-coatlayer contains carbon as a main component and 5 to 20 atom % ofhydrogen. For this reason, friction between the back-coat layer and theprotective layer decreases.

In the magnetic recording medium, the protective layer and the back-coatlayer are preferably formed by a plasma CVD method.

When the protective layer and the back-coat layer are formed by theplasma CVD method, the forming rates of the protective layer and theback-coat layer can be increased. For this reason, productivity can beimproved. In addition, when the protective layer and the back-coat layerare formed by the plasma CVD method, no binder is required in formationof the protective layer and the back-coat layer. Therefore, when theprotective layer and the back-coat layer are to be formed, anappropriate composition of the binder need not be found.

In the magnetic recording medium, the thickness of the protective layerpreferably falls within the range of 2 to 20 nm, and the thickness ofthe back-coat layer preferably falls within the range of 5 to 500 nm.

In this manner, the thickness of the protective layer is set to be 2 nmor more, the durability of the magnetic tape can be improved. However,when the thickness of the protective layer is excessively large, anoutput is attenuated. For this reason, the thickness of the protectivelayer is preferably set to be 20 nm or less. More specifically, asdescribed above, when the thickness of the protective layer is setwithin the range of 2 to 20 nm, a magnetic recording medium having bothexcellent durability and high output can be obtained.

In the magnetic recording medium, a fluorine-based lubricant ispreferably coated on at least one of the protective layer and theback-coat layer. As the fluorine-based lubricant, for example,perfluoropolyether, an ester (e.g., C₁₈ H₃₅ COO--CH₂ --CF₂ O--(C₂F₄₀)_(m) --(CF₂ O)_(n) --CH₂ --COOC₁₈ H₃₅) between a carbonic acid andperfluoropolyether, and ester of perfluoroalkylcarboxylate,perfluoroalkylester carboxylate, perfluoroalkylesterperfluoroalkylcarboxylate, perfluoroalkylamide carboxylate, orperfluoroalkylamide perfluoroalkylcarboxylate, or a derivative thereofcan be used.

In this manner, a fluorine-based lubricant is coated on the protectivelayer or the back-coat layer, the friction coefficient of the protectivelayer or the back-coat layer decreases, and further improvement onsliding characteristics of the magnetic recording medium can beachieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a main part of a magnetic recordingmedium to which the present invention is applied;

FIG. 2 is a typical view showing an arrangement of a depositionapparatus;

FIG. 3 is a typical view showing an arrangement of a plasma CVDapparatus; and

FIG. 4 is a graph showing a result obtained by measuring the FT--IRspectrum of a protective layer.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

An embodiment to which the present invention is applied will bedescribed below with reference to the accompanying drawings. As a matterof course, the present invention is not limited to the above embodiment.

A magnetic recording medium according to this embodiment is a magnetictape used in an 8-mm VTR. As shown in FIG. 1, in the magnetic recordingmedium, a magnetic layer 2 is formed on the front surface of a film-likenon-magnetic support member 1, a protective layer 3 is formed on themagnetic layer 2, and a back-coat layer 4 is formed on the rear surfaceof the non-magnetic support member 1.

In the magnetic tape, the magnetic layer 2 is formed in such a mannerthat a ferromagnetic metal material containing Co as a main component isdeposited on the front surface of the non-magnetic support member 1. Theprotective layer 3 and the back-coat layer 4 are formed by a plasma CVDapparatus.

In this case, the respective thin-film layers formed on the non-magneticsupport member 1 are formed in the order of the back-coat layer 4, themagnetic layer 2, and the protective layer 3. When the back-coat layer 4is formed before the magnetic layer 2 is formed, the back-coat layer 4is peeled by heat or the like generated in deposition of the magneticlayer 2 because the adhering force of the back-coat layer 4 is weak, orsome portions of the non-magnetic support member 1 are extremelycontracted due to heat. As a result, the surface of the back-coat layer4 disadvantageously becomes coarse.

However, as in this embodiment, since the back-coat layer 4 formed bythe plasma CVD apparatus has a strong adhering force, no problem isposed by heat or the like generated when the magnetic layer 2 isdeposited. For this reason, in this embodiment, the back-coat layer 4can be formed before the magnetic layer 2 is formed. Since the layersare formed in the above order, static electricity generated when themagnetic layer 2 is deposited is rapidly removed by the back-coat layer4 which has been formed. As a result, a magnetic tape having preferablecharacteristics can be obtained.

A deposition apparatus used in formation of the magnetic layer 2 will bedescribed below.

In this deposition apparatus, as shown in FIG. 2, a feeding roll 13rotated at a predetermined rate clockwise in FIG. 2 and a winding roll14 rotated at a predetermined rate clockwise in FIG. 2 are arranged in avacuum chamber 12 evacuated from exhaust ports 11 formed in the head andbottom portions of the vacuum chamber 12. The tape-like non-magneticsupport member 1 sequentially travels from the feeding roll 13 to thewinding roll 14.

A cylindrical can 15 having a diameter larger than that of each of therolls 13 and 14 is arranged at the middle portion where the non-magneticsupport member 1 travels from the feeding roll 13 to the winding roll14. The cylindrical can 15 is arranged such that the non-magneticsupport member 1 is drawn downward in FIG. 2, and is designed to berotated at a predetermined rate clockwise in FIG. 2. The feeding roll13, the winding roll 14, and the cylindrical can 15 are respectivelyconstituted by cylindrical shapes each having a length almost equal tothe width of the non-magnetic support member 1. A cooling unit (notshown) is arranged inside of the cylindrical can 15, so that thenon-magnetic support member 1 can be suppressed from being deformed byan increase in temperature of the non-magnetic support member 1.

The non-magnetic support member 1 is sequentially fed from the feedingroll 13, passes through the peripheral surface of the cylindrical can15, and is wound on the winding roll 14. Guide rolls 16 and 17 arerespectively arranged between the feeding roll 13 and the cylindricalcan 15 and between the cylindrical can 15 and the winding roll 14.

In this case, the guide rolls 16 and 17 are used such that thenon-magnetic support member 1 smoothly travels. The guide roll 16applies a predetermined tension to the non-magnetic support member 1traveling from the feeding roll 13 to the cylindrical can 15. Similarly,the guide roll 17 applies a predetermined tension to the non-magneticsupport member 1 traveling from the cylindrical can 15 to the windingroll 14.

In the vacuum chamber 12, a crucible 18 is arranged below thecylindrical can 15, and the crucible 18 is filled with a ferromagneticmetal material 19. The crucible 18 has a width almost equal to thelength of the cylindrical can 15.

On the other hand, an electron gun 20 for heating and vaporizing theferromagnetic metal material 19 in the crucible 18 is attached to theside wall portion of the vacuum chamber 12. The electron gun 20 isarranged at a position where an electron beam X emitted from theelectron gun 20 is irradiated onto the ferromagnetic metal material 19in the crucible 18. The ferromagnetic metal material 19 vaporized by theelectron gun 20 is formed on and adhered to the non-magnetic supportmember 1 traveling at a predetermined rate around the cylindrical can 15as the magnetic layer 2.

A shutter 21 is arranged between the cylindrical can 15 and the crucible18 and near the cylindrical can 15. The shutter 21 is formed to cover apredetermined region of the non-magnetic support member 1 traveling at apredetermined rate around the cylindrical can 15. The shutter 21 isformed such that the ferromagnetic metal material 19 vaporized asdescribed above is obliquely deposited on the non-magnetic supportmember 1 at an angle falling within a predetermined angle range. Inaddition, in the deposition, an oxygen gas is supplied onto the frontsurface of the non-magnetic support member 1 in the vacuum chamber 12through an oxygen gas supply port 22 formed through the side wallportion of the vacuum chamber 12. In this manner, the magneticcharacteristics, durability, and weatherability of the magnetic layer 2to be formed can be improved.

The vacuum deposition is performed in the following manner. Morespecifically, the vacuum chamber 12 is kept at a degree of vacuum of,e.g., 1×10⁻⁴ Torr, and an oxygen gas is supplied at a rate of, e.g., 250cc/min from the oxygen gas supply port 22. The incident angle ofvaporized metal onto the non-magnetic support member 1 is set within therange of, 45° to 90°. The magnetic layer 2 is formed to have a thicknessof, e.g., 200 nm. As the ferromagnetic metal material 19 used as avaporization source, a ferromagnetic metal containing Co as a maincomponent, e.g., Co₈₀ --Ni₂₀ (numerical values represent compositions bywt %) is used.

The magnetic layer 2 may be formed by an ion-plating method in which theferromagnetic metal material is vaporized in discharge, a sputteringmethod in which glow discharge is generated in an atmosphere containingargon as a main component, and atoms on the ferromagnetic metal materialsurface are knocked on by argon ions generated by the glow discharge, orthe like.

A plasma CVD apparatus used in formation of the protective layer 3 andthe back-coat layer 4 will be described below.

This plasma CVD apparatus is a hollow-anode type plasma CVD apparatus.As shown in FIG. 3, in a vacuum chamber 30 kept at a degree of vacuum ofabout 10⁻² to 10⁻³ Pa, the tape-like non-magnetic support member 1serving as an object to be processed sequentially travels from a feedingroll 31 rotated at a predetermined rate clockwise in FIG. 3 and awinding roll 32 rotated at a predetermined rate clockwise in FIG. 3.

A cylindrical can 33 having a diameter larger than that of each of therolls 31 and 32 is arranged at the middle portion where the non-magneticsupport member 1 travels from the feeding roll 31 to the winding roll 32to draw the non-magnetic support member 1 downward in FIG. 3. Thecylindrical can 33 are rotated at a predetermined rate clockwise in FIG.3.

The feeding roll 31, the winding roll 32, and the cylindrical can 33 arerespectively constituted by cylindrical shapes each having a lengthalmost equal to the width of the non-magnetic support member 1. Acooling unit (not shown) is arranged inside of the cylindrical can 33,so that the non-magnetic support member 1 can be suppressed from beingdeformed by an increase in temperature of the non-magnetic supportmember 1.

The non-magnetic support member 1 is sequentially fed from the feedingroll 31, passes through the peripheral surface of the cylindrical can33, and is wound on the winding roll 32. In this case, guide rolls 34and 35 are respectively arranged between the feeding roll 31 and thecylindrical can 33 and between the cylindrical can 33 and the windingroll 32.

These guide rolls 34 and 35 are used such that the non-magnetic supportmember 1 smoothly travels. The guide roll 34 applies a predeterminedtension to the non-magnetic support member 1 traveling from the feedingroll 31 to the cylindrical can 33. Similarly, the guide roll 35 appliesa predetermined tension to the non-magnetic support member 1 travelingfrom the cylindrical can 33 to the winding roll 32.

A pair of electrodes 36 and 37 are arranged below the cylindrical can 33in the vacuum chamber 30. The electrodes 36 and 37 respectivelyconstitute boxes (hollows) each having an almost rectangular section. Anelectrode arranged opposite to the cylindrical can 33 serves as theanode electrode 36, and an electrode arranged to surround the anodeelectrode 36 through an insulator serves as the cathode electrode 37. Inthis case, the cathode electrode 37 is grounded, and has an upper endportion which is opened along the peripheral surface of the cylindricalcan 33.

The anode electrode 36 is connected to an AC power supply 39 arrangedoutside the vacuum chamber 30 through a capacitor 38. A source gassupply tube 40 arranged through the outside wall of the vacuum chamber30 is attached to the anode electrode 36. The source gas supply tube 40is hollow, and source gases such as an ethylene gas or a hydrogen gasare supplied through the source gas supply tube 40.

In film formation, a predetermined voltage is applied from the AC powersupply 39 to the anode electrode 36 to generate discharge between thecylindrical can 33 and the anode electrode 36, and the source gases aresupplied into the space between the anode electrode 36 and thecylindrical can 33 through the source gas supply tube 40. At this time,the pressure of the space between the anode electrode 36 and thecylindrical can 33 is set to be high, i.e., about 80 Pa. Chemicalreactions such as decomposition, synthesis, and the like of the sourcegases are caused by a plasma generated by the above discharge. As aresult, film formation is performed on the non-magnetic support member 1passing through the space between the anode electrode 36 and thecylindrical can 33.

In this film formation, about 60 to 85% of argon gas is mixed in thesource gases. For this reason, stable discharge can be generated, RFpower can be increased, and a self-bias voltage applied across theelectrodes 36 and 37 can be increased. When the mixing rate of the argongas is increased while the RF power is kept constant, the self-biasvoltage can also be increased. When the self-bias voltage is increased,a film having high hardness can be formed, and the durability andtraveling properties of the manufactured magnetic tape can be improved.

The protective layer and the back-coat layer formed by the above plasmaCVD apparatus are not easily peeled because the layers have strongadhering forces, and can be used as a preferable protective layer and apreferable back-coat layer, respectively. In addition, the method offorming a protective layer or a back-coat layer using the above plasmaCVD apparatus has a film forming rate which is not lower than that of amethod of coating a back-coat layer material or a protective layermaterial on a film and drying it to form a back-coat layer or aprotective layer. For this reason, high productivity can be obtained.

Magnetic tapes on which protective layers and back-coat layers wereformed using the above plasma CVD apparatus were manufactured while thefilm formation conditions of the protective layers and the back-coatlayers were changed, and the characteristics of the magnetic tapes wereevaluated.

In order to examine an influence of the hydrogen content of a protectivelayer on the characteristics of a magnetic tape, a protective layer wasformed by using the above plasma CVD apparatus under the film formationconditions shown in Table 1, samples 1 to 5 in which the hydrogencontents of the protective layers are different from each other weremanufactured.

                  TABLE 1    ______________________________________                 PROTEC-                 TIVE                 LAYER    SOURCE GAS  DEGREE    INPUT        THICK-   ETHYLENE/   OF    POWER        NESS     HYDROGEN    VACUUM     W!           nm!      SCCM/SCCM!  Pa!    ______________________________________    SAMPLE 1                  200/0     60    SAMPLE 2                  200/50    60    SAMPLE 3            800      15       200/100   60    SAMPLE 4                  200/0     100    SAMPLE 5                   5/250    60    ______________________________________

The hydrogen contents of the protective layer formed under the filmformation conditions shown in Table 1 were measured.

In this case, in measurement of the hydrogen content, film formation wasperformed under the same condition as that of Sample 3, and the ratio ofhydrogen to oxygen was measured using a combustion method. Morespecifically, a carbon powder was made of the film formed under the samecondition as that of Sample 3, and the carbon powder was sufficientlydried. Thereafter, the weights of CO₂ and H₂ O generated when the carbonpowder was burned were measured, and (CO₂ weight/44):(H₂ O weight×2/18)was calculated. As a result, a ratio of 94:5 was obtained, and it wasunderstood that the hydrogen content was about 5 atom % under thecondition of Sample 3.

A film having a thickness of 50 nm was formed under the same conditionas that of Sample 3, and the Fourier-transform infrared ray (FT-IR)spectrum of the film was measured by a transmission-type FT-IR spectrummeasurement apparatus. The obtained result is shown in FIG. 4. At thistime, when the ratio of absorption by C--H bonds near a wave number of2,890 cm⁻¹ and absorption by C--C bonds near a wave number of 1,500 cm⁻¹i,e., the ratio of the area of a portion indicated by a hatched portionA in FIG. 4 and the area of a portion indicated by a hatched portion Bin FIG. 4, was calculated, a ratio of about 1:5 could be obtained.

As is apparent from these results, when the ratio of C--H bonds and C--Cbonds was 1:5, the hydrogen content was about 5 atom %. Therefore, withrespect to the protective layers except the protective layer of Sample3, the FT-IR spectra were measured to calculate the ratios of C--H bondsto C--C bonds, and hydrogen components were calculated by a proportionaldistribution method.

In order to examine the relationship between the hydrogen contentmeasured as described above and the effect of the protective layer,periods of durability time of the magnetic tapes on which the protectivelayers were formed under the film formation conditions shown in Table 1when a reproducing operation by a VTR is temporarily stopped, i.e.,periods of still durability time, were measured.

In this case, a recorder obtained by modifying a video tape recorder(CVD-1000: trade name of Sony Corp.) was used in the measurement. Themodification was performed as described below. That is, an algorithm forcanceling a still function for eight minutes was removed, the stilloperation was continued until the stop operation was instructed, and aquartz oscillator having a frequency of 7.6 MHz was attached immediatelybefore a recording amplifier. In this manner, the recorder was designedsuch that a single sine wave could be input.

Table 2 shows the results obtained by measuring the hydrogen contents ofthe protective layers of the respective samples and the results obtainedby measuring the still durabilities of the magnetic tapes.

                  TABLE 2    ______________________________________           HYDROGEN CONTENT            atom %!       STILL DURABILITY    ______________________________________    SAMPLE 1 20               24 hours or longer    SAMPLE 2 15               24 hours or longer    SAMPLE 3 5                24 hours or longer    SAMPLE 4 25               15 hours    SAMPLE 5 3                6 hours    ______________________________________

As is apparent from the results shown in Table 2, the durability of themagnetic tape is excellent when the hydrogen content was 5 to 20 atom %.More specifically, when a protective layer containing carbon as a maincomponent and 5 to 20 atom % of hydrogen is formed on a magnetic layer,the durability of the magnetic tape can be improved.

In order to examine an influence of the hydrogen content of a back-coatlayer on the characteristics of a magnetic tape, back-coat layers wereformed on the rear surfaces of non-magnetic support members under thefilm formation conditions shown in Table 3 to manufacture Samples 6 to10 in which the hydrogen contents of the back-coat layers were differentfrom each other.

                  TABLE 3    ______________________________________                     SOURCE GAS           INPUT POWER                     ETHYLENE/   DEGREE OF            W!       HYDROGEN    VACUUM  Pa!    ______________________________________    SAMPLE 6             200/0       60    SAMPLE 7             200/50      60    SAMPLE 8 800         200/100     60    SAMPLE 9             200/0       100    SAMPLE 10            200/250     60    ______________________________________

As in measurement of the hydrogen content of the protective layersdescribed above, the hydrogen contents of the back-coat layers of therespective samples shown in Table 3 were measured. In order to examinethe relationship between the hydrogen content of the back-coat layer andthe effect of the back-coat layer, level-down amounts of the respectivesamples obtained when a recorded signal was repetitively reproduced weremeasured.

At this time, measurement of the level-down amount was performed in thefollowing manner. That is, a sine wave having a frequency of 7.6 MHz wasinput to a still track once to perform a recording operation, andoutputs were sequentially detected. The degree of attenuation of theoutput due to the influence of a clog or the like was calculated in sucha manner that the first output was compared with the output obtainedafter shuttling was performed 100 times, and the rate of the attenuationwas defined as a level-down amount.

Table 4 shows the results obtained by measuring the hydrogen contents ofthe back-coat layers of the respective samples and the results obtainedby measuring the level-down amounts.

                  TABLE 4    ______________________________________    HYDROGEN CONTENT     atom %!            LEVEL-DOWN AMOUNT    ______________________________________    SAMPLE 6            20              -0.5db    SAMPLE 7            15              -0.5db    SAMPLE 8            5               -1.0db    SAMPLE 9            25              -6.0db    SAMPLE 10            3               -5.0db    ______________________________________

As is apparent from the results shown in Table 4, the level-down amountis preferably small when the hydrogen content is 5 to 20 atom %. Morespecifically, a back-coat layer containing when a protective layercontaining carbon as a main component and 5 to 20 atom % of hydrogen isformed on the rear surface of a non-magnetic member, the characteristicsof the magnetic tape can be improved.

In order to examine an influence of the thickness of a back-coat layeron the characteristics of a magnetic tape, as shown in Table 5, Samples11 to 17 in which the thicknesses of back-coat layers were differentfrom each other. In Samples 11 to 17, the protective layers were formedunder the same condition as that of Sample 2. The back-coat layers wereformed under the same condition as that of Sample 7 except that thethicknesses of the back-coat layers are different from each other. Morespecifically, in Samples 11 to 17, the thickness of each of all theprotective layers is 15 nm, and the hydrogen content of each of all theback-coat layers and each of the protective layers are 15 atom %.

In manufacture of Samples 11 to 17, the thicknesses of the back-coatlayers were controlled by changing a feeding rate of the non-magneticsupport member in formation of the back-coat layers. More specifically,when the thickness of the back-coat layer was decreased, film formationwas performed such that the feeding rate of the non-magnetic member fromthe feeding roll to the winding roll was increased. When the thicknessof the back-coat layer was increased, film formation was performed suchthat the feeding rate of the non-magnetic member from the feeding rollto the winding roll was lowered.

                  TABLE 5    ______________________________________           THICKNESS OF                     PROTECTIVE  HYDROGEN           BACK-COAT LAYER       CONTENT           LAYER     THICKNESS    atom %!    ______________________________________    SAMPLE 11             3    SAMPLE 12             5    SAMPLE 13             20          15          15    SAMPLE 14             70    SAMPLE 15             100    SAMPLE 16             500    SAMPLE 17             600    ______________________________________

The surface properties of the protective layer and back-coat layer ofeach sample shown in Table 5 were examined. In this case, evaluation ofthe surface properties was performed in the following manner. Morespecifically, by using a SUS303, friction coefficients at a temperatureof 40° C. and a humidity of 80% were measured on the front surface onwhich the protective layer was formed and on the rear surface on whichthe back-coat layer was formed. Evaluation of the surface properties ofa magnetic tape manufactured without forming a back-coat layer as SampleA was performed for comparison. Measurement results are shown in Table6.

                  TABLE 6    ______________________________________    FRICTION         FRICTION     THICKNESS    COEFFICIENT      COEFFICIENT  OF BACK-    OF FRONT         OS REAR      COAT LAYER    SURFACE          SURFACE       nm!    ______________________________________    SAMPLE 11            0.23         0.29         3    SAMPLE 12            0.23         0.22         5    SAMPLE 13            0.23         0.21         20    SAMPLE 14            0.23         0.21         70    SAMPLE 15            0.23         0.21         100    SAMPLE 16            0.23         0.21         500    SAMPLE 17            0.23         IMPOSSIBLE   600                         MEASUREMENT    SAMPLE A            IMPOSSIBLE   IMPOSSIBLE   0            MEASUREMENT  MEASUREMENT    ______________________________________

In this case, since film peeling occurred on the rear surface of Sample17 when the friction coefficient was measured, the friction coefficientcould not be measured. Since sticking occurred on Sample A when thefriction coefficient was measured, the friction coefficient could not bemeasured.

As is apparent from the results shown in Table 6, the magnetic tapes ofSamples 11 to 17 have small friction coefficients, have excellenttraveling properties. In this case, although it is generally said thatthe friction coefficient of the magnetic tape is especially preferablyset to be 0.25 or less, the friction coefficients in Samples 12 to 16are smaller then this value. It is understood that the magnetic tapes ofSamples 12 to 16 are specially preferable. Therefore, the thickness ofthe back-coat layer is specially preferably set within the range of 5 to500 nm.

Magnetic tapes were manufactured while the thicknesses of the protectivelayers were different from each other, and still durabilities weremeasured in the same manner as in the above measurement. At this time,the same formation conditions as the formation conditions of theprotective layer and the back-coat layer of Sample 16 were used exceptfor the thickness of the protective layer. A magnetic tape manufacturedwithout a protective layer was set as Sample B. With reference to anoutput obtained when the magnetic tape of Sample B, the output levels ofother magnetic tapes were measured. Measurement results are shown inTable 7.

                  TABLE 7    ______________________________________           PROTECTIVE           LAYER           THICKNESS STILL            nm!      DURABILITY  OUTPUT LEVEL    ______________________________________    SAMPLE B 0           2 hours     reference                                     value (0db)    SAMPLE 18             1.4         15 hours    -0.2db    SAMPLE 19             2           24 hours or -0.3db                         longer    SAMPLE 20             5           24 hours or -0.7db                         longer    SAMPLE 21             10          24 hours or -1.4db                         longer    SAMPLE 22             20          24 hours or -2.8db                         longer    SAMPLE 23             30          24 hours or -4.2db                         longer    ______________________________________

As is apparent from the results shown in Table 7, the durability of themagnetic tape is excellent when the protective layer has a thickness of2 nm or more. However, when the thickness of the protective layer isexcessively large, an output attenuates. More specifically, as shown inTable 7, when the thickness of the protective layer exceeds 20 nm, theoutput attenuates by 3 dB or more. Therefore, in consideration of therelationship between durability and high output, the thickness of theprotective layer is preferably set within the range of 2 to 20 nm.

In the magnetic recording medium according to the present invention, afluorine-based lubricant may be coated on at least one of the protectivelayer and the back-coat layer.

After the protective layer and the back-coat layer are formed asdescribed above, perfluoropolyether (trade name: Fonbline Z-DOL) iscoated as a fluorine-based lubricant on each of the protective layer andthe back-coat layer to have a thickness of about 2 nm. As a result, thefriction coefficient of the protective layer and the back-coat layerdecreased, and the sliding characteristics of the magnetic tape werefurther improved.

The material of the magnetic layer of the magnetic recording mediumaccording to the present invention is not limited to the above example,and any material which is used in a general deposition-type magneticrecording medium can be used. More specifically, for example, aferromagnetic metal such as Fe, Co, or Ni or a ferromagnetic alloy suchas Fe--Co, Co--Ni, Fe--Co--Ni, Fe--Cu, Co--Cu, Co--Au, Co--Pt, Mn--Bi,Mn--Al, Fe--Cr, Co--Cr, Ni--Cr, Fe--Co--Cr, Co--Ni--Cr, orFe--Co--Ni--Cr can be used.

A magnetic layer consisting of such a ferromagnetic metal material mayhave a single-layer structure or a multi-layer structure. When themagnetic layer is constituted by a multi-layer structure, anintermediate layer may be formed between respective layers to improvethe adhering force between the respective layers and to control ananti-magnetic force.

INDUSTRIAL APPLICABILITY

As is apparent from the above description, a magnetic recording mediumaccording the present invention has very excellent characteristics,preferable traveling properties, and high durability.

When a protective layer and a back-coat layer are formed by a plasma CVDmethod, a protective layer and a back-coat layer each having a strongadhering force and preferable characteristics can be formed.

In addition, according to the plasma CVD method, a film forming rate ishigh. For this reason, when a protective layer or a back-coat layer isformed by the plasma CVD method, productivity can be improved.

I claim:
 1. A magnetic recording medium comprising:a non-metallic support member having front and rear sides, the front side being covered by a ferromagnetic metal thin film, the ferromagnetic metal thin film being containing Co covered by a protective layer having a thickness ranging from about 2 nm to about 20 nm, the back side being covered by a back coat layer having a thickness ranging from about 5 nm to about 500 nm, the protective layer comprising hydrogen in an amount ranging from about 5 atom % to about 20 atom % and carbon, the protective layer being substantially free of any binder, the back coat layer comprising hydrogen in an amount ranging from about 5 atom % to about 20 atom % and carbon, the back coat layer being substantially free of any binder, the protective layer and the back coat layer being coated with a fluorine based lubricant selected from the group consisting of perfluoropolyether, an ester of a carbonic acid and perfluoropolyether, an ester of perfluoroalkylcarboxylate, perfluoroalkylester carboxylate, perfluoroalkylester perfluoroalkylcarboxylate, perfluoroalkylamide carboxylate and, perfluoroalkylamide perfluoroalkylcarboxylate.
 2. The magnetic recording medium of claim 1 wherein the hydrogen concentration of the protective layer is approximately equal to the hydrogen concentration of the back coat layer. 